In recent years, the development of miniaturized portable electronics has provided a need for a rechargeable lithium battery having a high capacity as well as a light weight. The capacity of the rechargeable lithium battery depends on the positive active materials. The electrochemical characteristics of the positive active materials influence the high-rate cycle characteristics and the capacity retention of the rechargeable lithium battery.
Manganese-based active materials such as LiMn.sub.2 O.sub.4, and LiMnO.sub.2, among all of the positive active materials, are the easiest to prepare, are less expensive than the other materials and have environmentally friendly characteristics. Among such manganese-based compounds, LiMn.sub.2 O.sub.4 is particularly stable for battery use and is thus attractive for electric vehicle applications.
LiMn.sub.2 O.sub.4 exhibits good room-temperature cycle life characteristics, but poor high-temperature cycle life characteristics. The manganese in LiMn.sub.2 O.sub.4 has an atomic value of 3.5, and thus it substantially has a formal charge of +3 and a formal charge of +4. Therefore, two types of Mn in LiMn.sub.2 O.sub.4 exist: Mn.sup.3+ and Mn.sup.4+. When the ambient temperature is raised, stable Mn.sup.4+ is not oxidized or reduced, but unstable Mn.sup.3+ is oxidized to Mn.sup.4+ or reduced to Mn.sup.2+. This is a disproportionate reaction and causes an abrupt loss of capacity at high temperatures. This capacity loss mostly occurs in initial charge-discharge cycles, for example, between the first and tenth charge-discharge cycles.
In addition, when a battery using a manganese-based material such as LiMn.sub.2 O.sub.4 is charged and discharged many times, particularly at high temperatures, a side reaction between the electrolyte and the manganese-based material occurs at the surface of the manganese-based material. It is believed that H.sub.2 O reacts with LiPF.sub.6 in the electrolyte to generate the strong acid HF, which attacks Mn presented at the surface of the manganese-based active material, and the attacked Mn is eluted into and dissolved in the electrolyte, so the active material disintegrates. This side reaction seriously deteriorates the cycle life characteristics of the battery.
Attempts to solve these problems have been performed by synthesizing a material including at least one equivalent of Li, or partially substituting oxygen (O) with fluoride (F) in a spinel manganese-based material. However, those methods do not effectively improve the cycle life characteristics of the battery, particularly at high temperatures.